The present disclosure relates to a controller for an internal combustion engine.
An internal combustion engine with a NOx storage reduction type catalyst installed in the exhaust passage is known. In such an internal combustion engine, NOx in the exhaust gas is stored in the catalyst, when combustion is performed at an air-fuel ratio leaner than a stoichiometric air-fuel ratio. When the air-fuel ratio of the air-fuel mixture burned in the internal combustion engine temporarily becomes a rich air-fuel ratio, NOx stored by the catalyst is reduced and purified. Thus, the emission of NOx to the outside air is reduced.
In the NOx storage reduction type catalyst, sulfur oxide (SOx) in the exhaust gas and NOx are stored as a sulfur compound such as sulfide salt. As the deposition of sulfur compounds progresses, the NOx storage capacity of the catalyst decreases and sulfur poisoning occurs.
Therefore, in the internal combustion engine equipped with the catalyst, as described in International Publication WO 2010/116535 and the like, a poison release control for reducing the sulfur compound deposited on the catalyst is performed. In the poison release control, fuel as a reducing agent is supplied to the exhaust gas flowing into the catalyst in the state in which the temperature of the catalyst is increased to a temperature (for example, 600° C.) required for desorption of sulfur compounds. As a result, a desorption process of desorbing and reducing the sulfur compounds deposited on the catalyst is performed. Further, in order to suppress the excessive rise in temperature of the catalyst due to the reduction reaction, after the desorption process is performed for a specified time, a pausing process of pausing the supply of fuel to the exhaust gas is performed. When the desorption process and the pause treatment are alternately repeated by the poison release control, the sulfur compound deposited on the catalyst gradually decreases.
In the desorption process, when fuel is supplied to the exhaust gas from a direct injection valve, which directly injects fuel into the cylinder, the air-fuel ratio of the exhaust gas flowing into the catalyst is made richer than the stoichiometric air-fuel ratio. During execution of the poison release control, the temperature of the exhaust gas increases to a temperature necessary for desorption of the sulfur compound. Therefore, the temperature of the addition valve provided in the exhaust passage also increases. Therefore, there is a risk of the occurrence of thermal damage to the addition valve. Thus, similarly to the apparatus described in Japanese Patent No. 4922899, in order to avoid an increase in the temperature of the addition valve in the exhaust gas purification apparatus, it is preferable to execute the cooling fuel addition, in which fuel is injected from the addition valve to cool the addition valve.
However, if the cooling fuel addition is executed during the execution of the desorption process, the amount of fuel contained in the exhaust gas becomes excessive, and the air-fuel ratio of the exhaust gas flowing into the catalyst becomes excessively rich. For this reason, a phenomenon in which unburned fuel in the exhaust gas passes through the catalyst, that is, a so-called HC slip may occur. If the cooling fuel addition is not executed during the execution of desorption process, the temperature of the addition valve increases, and there is a risk of the occurrence of thermal damage to the addition valve.